A metal-oxide-semiconductor N channel field-effect transistor (FET) with a weak-P type channel region and an N channel charge-coupled device (CCD) both experience a similar phenomenon at cryogenic temperatures: when the control gate potential is increased there is still a residual potential barrier to electron flow through the conduction band. That potential barrier lies between the N.sup.+ source and the P type region beneath the control gate. The potential barrier happens because electrons within the transition region between the source and channel feel the potential of the source more strongly than the potential of the more distant control gate which is on the other side of the oxide layer above the channel. This residual potential barrier prevents an FET from "turning on" and similarly prevents "charge-injection" into a CCD.
If the device were sufficiently warm then electrons would be thermally excited over the potential barrier as is the case for almost all FETs and CCDs at room temperature. However, at the cryogenic temperatures normally required for infrared detectors, the device temperature is often too low for thermal excitation over the potential barrier and thus the barrier effectively blocks any current flow.
Once sufficient current has been established in the FET channel then the FET will heat up by joule heating within the channel region and thus maintain a sufficiently high temperature that continued operation over the potential barrier is possible. Similarly, once charge is successfully injected into the first "bucket" of a CCD, it can then be readily moved to other buckets because there are no residual potential barriers between successive buckets. Therefore, several means of "turning on" a cryogenic FET or a cryogenic CCD have been tried.
One prior art turn-on technique is to apply a large potential to the FET's control gate. This lowers the height of the potential barrier in the same manner as when a high voltage potential is used to create field emission from a cold cathode in a vacuum. Because of the limited amount of field that can be placed cross the thin dielectric between the gate and the channel region, this technique can only change the potential barrier height by a small amount; thus not all types of cryogenically cooled FETs can accommodate a high enough gate potential to be turned on by this technique.
Once the FET has been so turned on, the FET's gate can then be returned to its normal operating range. If the current is turned completely off again, either intentionally or unintentionally, the gate will again have to be subjected to the above-mentioned high potential to again turn the FET on. This erratic off-again, on-again behavior is undesirable.
A second prior art technique is to heat the FET (or CCD). This results in electronics which are thermally incompatible with the infrared detectors with which the electronics is designed to work. The heated electronics must be on a separate chip (substrate) which is thermally isolated from the detectors and the detector heat sink. This also necessitates a chip-to-chip "cable" connection. Thus this second technique is complicated to implement and the extra chip-to-chip wiring is often a source of unreliability.
A third prior art technique is to shine a light onto the FET (or CCD). The absorbed light generates some free electrons which then flow through the channel, thus heating the FET until it turns on. The problem of localizing the light at the electronics and preventing stray light from interacting with the infrared detectors makes this third method incompatible with having the detectors and the electronics adjacent one another on the same chip, again requiring separate chips and chip-to-chip wiring.